Effects of internal and external velocity on the perceived direction of the double-drift illusion

Using Illusions to Track the Emergence of Visual Perception

Everybody loves illusions. At times, the content on the internet seems to be mostly about illusions-shoes, dresses, straight lines looking bent. This attraction has a long history. Almost 2,000 years ago, Ptolemy marveled at how the sail of a distant boat could appear convex or concave. This sense of marvel continues to drive our fascination with illusions; indeed, few other corners of science can boast of such a large reach. However, illusions not only draw in the crowds; they also offer insights into visual processes. This review starts with a simple definition of illusions as conflicts between perception and cognition, where what we see does not agree with what we believe we should see. This mismatch can be either because cognition has misunderstood how perception works or because perception has misjudged the visual input. It is the perceptual errors that offer the chance to track the development of perception across visual regions. Unfortunately, the effects of illusions in different brain regions cannot be isolated in any simple way: Top-down projections from attention broadcast the expected perceptual properties everywhere, obscuring the critical evidence of where the illusion and perception emerge. The second part of this review then highlights the roadblocks to research raised by attention and describes current solutions for accessing what illusions can offer.

Different extrapolation of moving object locations in perception, smooth pursuit, and saccades

The ability to accurately perceive and track moving objects is crucial for many everyday activities. In this study, we use a "double-drift stimulus" to explore the processing of visual motion signals that underlie perception, pursuit, and saccade responses to a moving object. Participants were presented with peripheral moving apertures filled with noise that either drifted orthogonally to the aperture's direction or had no net motion. Participants were asked to saccade to and track these targets with their gaze as soon as they appeared and then to report their direction. In the trials with internal motion, the target disappeared at saccade onset so that the first 100 ms of the postsaccadic pursuit response was driven uniquely by peripheral information gathered before saccade onset. This provided independent measures of perceptual, pursuit, and saccadic responses to the double-drift stimulus on a trial-by-trial basis. Our analysis revealed systematic differences between saccadic responses, on one hand, and perceptual and pursuit responses, on the other. These differences are unlikely to be caused by differences in the processing of motion signals because both saccades and pursuits seem to rely on shared target position and velocity information. We conclude that our results are instead due to a difference in how the processing mechanisms underlying perception, pursuit, and saccades combine motor signals with target position. These findings advance our understanding of the mechanisms underlying dissociation in visual processing between perception and eye movements.

The double-drift illusion biases the marmoset oculomotor system

The double-drift illusion has two unique characteristics: The error between the perceived and physical position of the stimulus grows over time, and saccades to the moving target land much closer to the physical than the perceived location. These results suggest that the perceptual and saccade targeting systems integrate visual information over different time scales. Functional imaging studies in humans have revealed several potential cortical areas of interest, including the prefrontal cortex. However, we currently lack an animal model to study the neural mechanisms of location perception that underlie the double-drift illusion. To fill this gap, we trained two marmoset monkeys to fixate and then saccade to the double-drift stimulus. In line with human observers for radial double-drift trajectories with fast internal motion, we find that saccade endpoints show a significant bias that is, nevertheless, smaller than the bias seen in human perceptual reports. This bias is modulated by changes in the external and internal speeds of the stimulus. These results demonstrate that the saccade targeting system of the marmoset monkey is influenced by the double-drift illusion.

Measuring the double-drift illusion and its resets with hand trajectories

If a Gabor pattern drifts in one direction while its internal texture drifts in the orthogonal direction, its perceived position deviates further and further away from its true path. We first evaluated the illusion using manual tracking. Participants followed the Gabor with a stylus on a drawing tablet that coincided optically with the horizontal monitor surface. Their hand and the stylus were not visible during the tracking. The magnitude of the tracking illusion corresponded closely to previous perceptual and pointing measures indicating that manual tracking is a valid measure for the illusion. This allowed us to use it in a second experiment to capture the behavior of the illusion as it eventually degrades and breaks down in single trials. Specifically, the deviation of the Gabor stops accumulating at some point and either stays at a fixed offset or resets toward the veridical position. To report the perceived trajectory of the Gabor, participants drew it after the Gabor was removed from the monitor. Resets were detected and analyzed and their distribution matches neither a temporal nor a spatial limit, but rather a broad gamma distribution over time. This suggests that resets are triggered randomly, about once per 1.3 seconds, possible by extraneous distractions or eye movements.